How to duplicate 1st line

How to duplicate 1st line

Hi,

because of a noise in the 1st line of my video, I want to duplicate the
2nd line to the first. I tried this and got an error about invalid
horizontal crop value:
ffmpeg -i in.vob -vf
"split[in1][in2];[in1]crop=in_w:1:0:1[top];[in2]crop=in_w:in_h-1:0:1[main];[top][main]
vstack"out_fill-top_.mp4

I guess, the problem is, that the input file is interlaced. So I tried
the following:
ffmpeg -i in.vob -vf
"split[in1][in2];[in1]crop=in_w:2:0:2[top];[in2]crop=in_w:in_h-2:0:2[main];[top][main]
vstack"out_fill-top_.mp4
This works, but results in visible artefacts in the first 2 lines.

Because the resulting mp4 video is progressive anyway, how could I get
what I want (only 1 line should be duplicated)?